Based on the Master's familiarity with the event, he was most likely involved, possibly opening the time rift himself.

Evelina, who could see the future, mentioned the Medusa Cascade when commenting on the Doctor, saying "Your real name is hidden. It burns in the stars, in the Cascade of Medusa herself". (DW: The Fires of Pompeii)

The Medusa Cascade was also the place where Davros's New Dalek Empire moved 27 planets The planets could not be normally found as they were out of sync from the rest of the universe by about a second. (DW: The Stolen Earth) He placed them there so that when his Reality bomb was set off it could penetrate the space-time rift in the Cascade and spread the bomb's destructive force across every universe and all of existence.